High temperature shear stable nanographite dispersion lubricants with enhanced thermal conductivity and method for making

ABSTRACT

A process for producing a nanographite dispersion in a fluid wherein the thermal conductivity of the dispersion is enhanced from the base fluid by more than 10% for a 1% graphite dispersion. A high purity graphite with high crystallinity and reduced surface damage and oxidation is selected as the starting material. The starting material is subjected to a process of wet media milling in the presence of dispersant and solvent fluid. The mill temperature is controlled to control and reduce surface damage to yield a nanographite with flake shape and controlled aspect ratio until a particle size average of 300 nm diameter and 50 nm is obtained. The process recycles a portion of the milled material to increase the ratio of small particle distribution to large particles in an intermediate product with small and large particle bi-modal distribution. The large particle distribution is removed by a separation process such as centrifugation or filtration.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional Application Ser. No. 60/800,557 filed on May 15, 2006 and U.S. application Ser. No. 11/796,708 filed on Apr. 27, 2007 claiming priority from Provisional Application Ser. No. 60/795,814 filed on Apr. 27, 2006 and claims priority from 11/370,118 filed on Mar. 7, 2006 claiming priority from PCT/US06/001675 filed on Jan. 17, 2006 claiming priority from Provisional Application Ser. No. 60/644,042 filed on Jan. 14, 2005 all of which are incorporated by reference herein in their entirety. Reference to documents made in the specification is intended to result in such patents or literature cited are expressly incorporated herein by reference, including any patents or other literature references cited within such documents as if fully set forth in this specification.

TECHNICAL FIELD

The technical field of this invention is a process for making nanographite dispersions.

BACKGROUND OF THE INVENTION

Thermally conductive nano-sized graphite particles can only be produced under specific processing conditions. Heretofore the common processing for making graphite particles (larger than the particles disclosed herein, typically with average particle size 0.8 microns and above) has included dry milling and other milling processes that render the final particles low in thermal conductivity. For example, the commercially available samples from Acheson, Inc. have a thermal conductivity for 1% graphite in oil dispersions insignificantly greater than the oils without graphite (typically 0.13 to 0.14 W/mK) By the methods of this invention, flake, or more specifically, plate shaped nanographites are produced that have significantly higher thermal conductivity, and when dispersed in oil at 1 percent by weight, have an increased thermal conductivity of up to 10 to 15 percent as measured in W/mK, resulting in experimental values of typically from 0.165 to 0.17 WmK or more as compared to the 0.13 to 0.14 values for oil without the graphite particles control sample of the instant invention.

DESCRIPTION OF THE PRIOR ART

The starting material for making nanographites can be any high thermal conductivity graphite either in fibers. Previously, naturally formed “nano-graphites” have not been available in the marketplace at all. Recently, Hyperion Catalysis International, Inc. commercialized carbon nanotubes or so-called carbon fibrils, which have a graphitic content as set forth in U.S. Pat. No. 5,165,909 by Tennent et al. which issued in Nov. 24, 1992 and is hereby incorporated by reference. Carbon nanotubes are typically hollow graphite-like tubules having a diameter of generally several to several tens nanometers. They exist in the form either as discrete fibers or aggregate particles of nanofibers. The thermal conductivity of the Hyperion Catalysis International, Inc. material is not stated in their product literature; however, the potential of carbon nanotubes to convey thermal conductivity in a material is discussed in U.S. Pat. No. 5,165,909. Actual measurement of the thermal conductivity of the carbon fibrils they produced was not given in the patent, so the inference of thermal conductivity is general and somewhat speculative, based on graphitic structure.

Bulk graphite with high thermal conductivity is available from Poco Graphite as a graphite foam, with thermal conductivity higher than 100 W/mK, and is also available from the Carbide/Graphite Group, Inc. Graphite powders can be obtained from UCAR Carbon Company Inc., with thermal conductivity 10-500 W/mK, and typically >80 W/mK, and from Cytec Carbon Fibers LLC, with thermal conductivity 400-700 W/mK.

For many applications carbon nanotubes would be a preferable substitute, however, for stability in a high shear fluid flow, the nano graphites are stable whereas carbon nanotubes break up. The present invention provides a process to mill graphite into plate shape and the subsequently dispersions have a higher thermal conductivity and are shear stable at a high temperature of as much as 430° C. Typically, engine lubricant applications are subjected to temperatures in the 200 to 300° F. range.

SUMMARY OF THE INVENTION

The compositions, methods, or embodiments discussed are intended to be only illustrative of the invention disclosed by this specification. Variation on these particle nanomaterial, compositions, methods, or embodiments are readily apparent to a person of skill in the art based upon the teachings of this specification and are therefore intended to be included as part of the inventions disclosed herein.

Graphite materials having a controlled aspect ratio and high thermal conductivity are produced by the milling process of the present invention. The aspect ration must not be too high so as to be brittle in shear fields, but high enough to have enhanced thermal conductivity.

The bulk graphite from Poco Graphite is a graphite foam with a high thermal conductivity and can be obtained in bulk quantities and reduced to a nanometer-sized powder by the methods of this invention.

The use of bulk graphite foam or graphite powders as an inexpensive sources of nanomaterials for further processing into controlled aspect ratio and high thermal conductivity products has not been used before. The instant invention provides a method of reducing the graphite to produce an inexpensive nanomaterial having a particle size suitable for long term dispersion in various fluids, polymers, composites, gels, greases, plastics etc. and the method of dispersing same.

However, only certain processes will produce these nanographites with high thermal conductivity and the thermal conductivity can either be drastically reduced (to impart no benefit) or increased by the subsequent processing. Dry milling imparts too much change in surface characteristics and reduces thermal conductivity. It is critical to have the graphite powder further milled in a horizontal mill with liquid media (e.g. base oil or solvent) and to use dispersants during wet milling in order to prevent paste formation.

It is an object of the present invention to provide a process for producing a nanographite dispersion in a fluid wherein the thermal conductivity of the dispersion is enhanced from the base fluid by more than 10% for a 1% graphite dispersion.

It is an object of the present invention to provide a process wherein a high purity graphite with high crystallinity and reduced surface damage and oxidation is selected as the starting material.

It is an object of the present invention to provide a process of wet media milling the starting material in the presence of dispersant and solvent (fluid).

It is an object of the present invention to provide a process of controlling mill temperature to adjust the viscosity of the milling mixture to achieve high milling efficiency.

It is an object of the present invention to provide a process of milling until a nanographite with flake shape and controlled aspect ratio is achieved.

It is an object of the present invention to provide a process of milling until a particle size average of 300 nm diameter and 50 nm thick is reached or smaller.

It is an object of the present invention to provide a process of recycling the milled material throughly to increase the ration of small particle distribution to large particle in an intermediate product with small and large particle bi-modal distribution.

It is an object of the present invention to provide a process of removing the large particle distribution from the finished product by centrifugation or filtration.

It is an object of the present invention to provide a method of preparing a stable dispersion of the carbon nanomaterials in a liquid medium with the combined use of dispersants/surfactants and physical agitation for use in a lubricant.

It is an object of the present invention to provide a method in which the carbon nanomaterials are made from cost-effective high-thermal-conductivity graphite (with thermal conductivity higher than 80 W/mK).

It is an object of the present invention to provide a method of developing a method of forming carbon nanomaterials from inexpensive bulk graphite.

It is an object of the present invention to provide a method of utilizing carbon nanotube, graphite flakes, carbon fibrils, carbon particles and combinations thereof.

It is an object of the present invention to provide a method of using carbon nanotubes which are either single-walled, or multi-walled, with typical aspect ratio of 500-5000.

It is an object of the present invention whereby the carbon nanomaterial can optionally be surface treated to be hydrophilic at surface for ease of dispersing into the aqueous medium.

It is an object of the present invention to provide a method wherein the said dispersants/surfactants are soluble or highly dispersible in the said liquid medium.

Other objects, features, and advantages of the invention will be apparent with the following detailed description taken in conjunction with the accompanying drawings showing a preferred embodiment of the invention and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention will be had upon reference to the following description in conjunction with the accompanying drawings in which like numerals refer to like parts throughout the several views and wherein:

FIG. 1 is a scanning electron microscope microphotograph of the graphitic raw material obtained from UCAR GS4-E showing the material shaped as chunks;

FIG. 2 is a scanning electron microscope microphotograph showing the graphitic material of FIG. 1 after solvent milling illustrating the graphite nanoparticles shown as plate-like structure.

FIG. 3 is a graph showing rheological measurement of graphite dispersion;

FIG. 4 is a graph showing rheological measurement of graphite dispersion;

FIG. 5 is an enlarged section taken from the microphotograph of FIG. 1;

FIG. 6 is an enlarged section taken from a microphotograph after further processing of the nanographite material of FIG. 1;

FIG. 7 is an enlarged section taken from the microphotograph of FIG. 2 resulting from additional processing of the nanographite material shown in FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following examples of the process is illustrated with nanographites dispersed in lubricant formulations.

The term dispersant in the instant invention refers to a surfactant added to a medium to promote uniform suspension of extremely fine solid particles, often of colloidal size. In the examples provided the dispersant is generally a long chain oil soluble or dispersible compound that attaches to the particles and disperses them. The term surfactant in the instant invention refers to any chemical compound that reduces surface tension of a liquid when dissolved into it, or reduces interfacial tension between two liquids or between a liquid and a solid. It is usually, but not exclusively, a long chain molecule comprised of two moieties; a hydrophilic moiety and a lipophilic moiety. The hydrophilic and lipophilic moieties refer to the segment in the molecule with affinity for water, and that with affinity for oil, respectively. These two terms, dispersant and surfactant, are mostly used interchangeably in the instant invention for often a surfactant has dispersing characteristics and many dispersants have the ability to reduce interfacial tensions.

The ashless dispersants used in the examples contain a lipophilic hydrocarbon group and a polar functional hydrophilic group. The polar functional group can be of the class of carboxylate, ester, amine, amide, imine, imide, hydroxyl, ether, epoxide, phosphorus, ester carboxyl, anhydride, or nitrile. The lipophilic group can be oligomeric or polymeric in nature, usually from 70 to 200 carbon atoms to ensure oil solubility. Hydrocarbon polymers treated with various reagents to introduce polar functions include products prepared by treating polyolefins such as polyisobutene first with maleic anhydride, or phosphorus sulfide or chloride, or by thermal treatment, and then with reagents such as polyamine, amine, and ethylene oxide.

Of these ashless dispersants the ones typically used include N-substituted polyisobutenyl succinimides and succinates, alkyl methacrylate-vinyl pyrrolidinone copolymers, alkyl methacrylate-dialkylaminoethyl methacrylate copolymers, alkyl methacrylate-polyethylene glycol methacrylate copolymers, and polystearamides. Preferred oil-based dispersants include dispersants from the chemical classes of alkylsuccinimide, succinate esters, high molecular weight amines, MANNICH base and phosphoric acid derivatives. Some specific examples are polyisobutenyl succinimide-polyethylenepolyamine, polyisobutenyl succinic ester, polyisobutenyl hydroxybenzyl-polyethylenepolyamine, bis-hydroxypropyl phosphorate. Commercial dispersants suitable include LUBRIZOL 890 (an ashless PIB succinimide), LUBRIZOL 6420 (a high molecular weight PIB succinimide), and ETHYL HITEC 646 (a non-boronated PIB succinimide), and ORONITE OLOA 12002 (succinimide). Preferred dispersants include PIB Succinimide and a dispersant VI improver olefin copolymer such as ORONITE OLOA 19075.

Furthermore, the carbon nanomaterial dispersion can be pre-sheared in a turbulent flow such as a nozzle, a high pressure fuel injector, an ultrasonic device, or a mill in order to achieve a stable viscosity. This may be especially desirable in the case where carbon nanotubes with high aspect ratio are used as the graphite source, since they, even more than spherical particles, will thicken the fluid but loose viscosity when exposed in turbulent flows. Pre-shearing, for example by milling, sonicating, or passing through a small orifice, such as in a fuel injector, is a particularly effective way to disperse the particles and to bring them to a stable size so that their viscosity increasing effect will not change upon further use.

The milling process itself, or other pre-shearing process, can have a rather dramatic effect on the long term dispersion stability.

A novel method has been developed whereby graphite particles are milled to form a thick pasty liquid of particles with mean size less than 500 nanometers in diameter and typically 300 nm plus or minus 200 nm in diameter and 50 nm plus or minus 30 nm in thickness. It is expected that there are many ways of process that would produce similar particle sizes but destroy the thermal conductivity of the particles because the energy input causes surface damage such that the particle structure becomes less crystalline, more amorphous and also has a chunk-like shape instead of a platelet shape. For example, if one takes high thermal conductivity powders produced in a jet mill and further dry mills these powders, a material of low thermal conductivity would be expected to result due to high surface temperatures produced in the dry milling process.

Oil Base Stocks

The petroleum liquid medium can be any petroleum distillates or synthetic petroleum oils, greases, gels, or oil-soluble polymer composition. More typically, it is the mineral base stocks or synthetic base stocks used in the lube industry, e.g., Group I (solvent refined mineral oils), Group II (hydrocracked mineral oils), Group III (severely hydrocracked oils, sometimes described as synthetic or semi-synthetic oils), Group IV (polyalphaolefins), and Group VI (esters, naphthenes, and others). One preferred group includes the polyalphaolefins, synthetic esters, and polyalkylglycols.

Synthetic lubricating oils include hydrocarbon oils and halo-substituted hydrocarbon oils such as polymerized and interpolymerized olefins (e.g., polybutylenes, polypropylenes, propylene-isobutylene copolymers, chlorinated polybutylenes, poly(1-octenes), poly(1-decenes), etc., and mixtures thereof; alkylbenzenes (e.g., dodecylbenzenes, tetradecylbenzenes, dinonylbenzenes, di-(2-ethylhexyl)benzenes, etc.); polyphenyls (e.g., biphenyls, terphenyls, alkylated polyphenyls, etc.), alkylated diphenyl, ethers and alkylated diphenyl sulfides and the derivatives, analogs and homologs thereof and the like.

Alkylene oxide polymers and interpolymers and derivatives thereof where the terminal hydroxyl groups have been modified by esterification, etherification, etc. constitute another class of known synthetic oils.

Another suitable class of synthetic oils comprises the esters of dicarboxylic acids (e.g., phtalic acid, succinic acid, alkyl succinic acids and alkenyl succinic acids, maleic acid, azelaic acid, suberic acid, sebacic acid, fumaric acid, adipic acid, alkenyl malonic acids, etc.) with a variety of alcohols (e.g., butyl alcohol, hexyl alcohol, dodecyl alcohol, 2-ethylhexyl alcohol, ethylene glycol diethylene glycol monoether, propylene glycol, etc.). Specific examples of these esters include dibutyl adipate, di(2-ethylhexyl) sebacate, di-hexyl fumarate, dioctyl sebacate, diisooctyl azelate, diisodecyl azealate, dioctyl phthalate, didecyl phthalate, dicicosyl sebacate, the 2-ethylhexyl diester of linoleic acid dimer, the complex ester formed by reacting one mole of sebacic acid with two moles of tetraethylene glycol and two moles of 2-ethylhexanoic acid, and the like.

Esters useful as synthetic oils also include those made from C (5) to C (12) monocarboxylic acids and polyols and polyol ethers such as neopentyl glycol, trimethylolpropane, pentaerythritol, dipentaerythritol, tripentaerythritol, etc. Other synthetic oils include liquid esters of phosphorus-containing acids (e.g., tricresyl phosphate, trioctyl phosphate, diethyl ester of decylphosphonic acid, etc.), polymeric tetrahydrofurans and the like.

Polyalphaolefins (PAO), useful in the present invention include those sold by BP Amoco Corporation as DURASYN fluids, those sold by Exxon-Mobil Chemical Company, (formerly Mobil Chemical Company) as SHF fluids, and those sold by Ethyl Corporation under the name ETHYLFLO, or ALBERMARLE. PAO's include the ETHYL-FLOW series by Ethyl Corporation, Albermarle Corporation, including ETHYL-FLOW 162, 164, 166, 168, and 174, having varying viscosity from about 2 to about 460 centistokes.

MOBIL SHF-42 from Exxon-Mobil Chemical Company, EMERY 3004 and 3006, and Quantum Chemical Company provide additional polyalphaolefins base stocks. For instance, EMERY 3004 polyalphaolefin has a viscosity of 3.86 centistokes (cSt) at 212° F. (100° C.) and 16.75 cSt at 104° F. (40° C.). It has a viscosity index of 125 and a pour point of −98° F. and it also has a flash point of about 432° F. and a fire point of about 478[deg] F. Moreover, EMERY 3006 polyalphaolefin has a viscosity of 5.88 cSt at +212° F. and 31.22 cSt at +104° F. It has a viscosity index of 135 and a pour point of −87° F.

Additional satisfactory polyalphaolefins are those sold by Uniroyal Inc. under the brand SYNTON PAO-40, which is a 40 centistoke polyalphaolefin.

It is contemplated that Gulf Synfluid 4 cSt PAO, commercially available from Gulf Oil Chemicals Company, a subsidiary of Chevron-Texaco Corporation, which is similar in many respects to EMERY 3004 may also be utilized herein. MOBIL SHF-41 PAO, commercially available from Mobil Chemical Corporation, is also similar in many respects to EMERY 3004.

Especially useful are the polyalphaolefins will have a viscosity in the range of up to 100 centistoke at 100[deg] C., with viscosity of 2 and 10 centistoke being more preferred.

The most preferred synthetic based oil ester additives are polyolesters and diesters such as di-aliphatic diesters of alkyl carboxylic acids such as di-2-ethylhexylazelate, di-isodecyladipate, and di-tridecyladipate, commercially available under the brand name EMERY 2960 by Emery Chemicals, described in U.S. Pat. No. 4,859,352 to Waynick. Other suitable polyolesters are manufactured by Mobil Oil. MOBIL polyolester P-43, NP343 containing two alcohols, and Hatco Corp. 2939 are particularly preferred.

Diesters and other synthetic oils have been used as replacements of mineral oil in fluid lubricants. Diesters have outstanding extreme low temperature flow properties and good residence to oxidative breakdown.

The diester oil may include an aliphatic diester of a dicarboxylic acid, or the diester oil can comprise a dialkyl aliphatic diester of an alkyl dicarboxylic acid, such as di-2-ethyl hexyl azelate, di-isodecyl azelate, di-tridecyl azelate, di-isodecyl adipate, di-tridecyl adipate. For instance, Di-2-ethylhexyl azelate is commercially available under the brand name of EMERY 2958 by Emery Chemicals.

Also useful are polyol esters such as EMERY 2935, 2936, and 2939 from Emery Group of Henkel Corporation and HATCO 2352, 2962, 2925, 2938, 2939, 2970, 3178, and 4322 polyol esters from Hatco Corporation, described in U.S. Pat. No. 5,344,579 to Ohtani et al. and MOBIL ESTER P 24 from Exxon-Mobil Chemical Company. Esters made by reacting dicarboxylic acids, glycols, and either monobasic acids or monohydric alcohols like EMERY 2936 synthetic-lubricant base stocks from Quantum Chemical Corporation and MOBIL P 24 from Exxon-Mobil Chemical Company can be used. Polyol esters have good oxidation and hydrolytic stability. The polyol ester for use herein preferably has a pour point of about −100° C. or lower to 40° C. and a viscosity of about 2 to 100 centistoke at 100° C.

A hydrogenated oil is a mineral oil subjected to hydrogennation or hydrocracking under special conditions to remove undesirable chemical compositions and impurities resulting in a base oil having synthetic oil component and properties. Typically the hydrogenated oil is defined by the American Petroleum Institute as a Group III base oil with a sulfur level less than 0.03 with saturates greater than or equal to 90 and a viscosity index of greater than or equal to 120. Most useful are hydrogenated oils having a viscosity of from 2 to 60 CST at 100 degrees centigrade. The hydrogenated oil typically provides superior performance to conventional motor oils with no other synthetic oil base. The hydrogenated oil may be used as the sole base oil component of the instant invention providing superior performance to conventional mineral oil bases oils or used as a blend with mineral oil and/or synthetic oil. An example of such an oil is YUBASE-4.

When used in combination with another conventional synthetic oil such as those containing polyalphaolefins or esters, or when used in combination with a mineral oil, the hydrogenated oil may be utilized as the oil base stock in an amount of up to 100 percent by volume, more preferably from about 10 to 80 percent by volume, more preferably from 20 to 60 percent by volume and most preferably from 10 to 30 percent by volume of the base oil composition.

A Group I or II mineral oil basestock may be incorporated in the present invention as a portion of the concentrate or a basestock to which the concentrate may be added. Preferred as mineral oil base stocks are the ASHLAND 325 Neutral defined as a solvent refined neutral having a SABOLT UNIVERSAL viscosity of 325 SUS @ 100° F. and ASHLAND 100 Neutral defined as a solvent refined neutral having a SABOLT UNIVERSAL viscosity of 100 SUS @ 100° F., manufactured by the Marathon Petroleum corporation.

Other acceptable petroleum-base fluid compositions include white mineral, paraffinic and MVI naphthenic oils having the viscosity range of about 20-400 centistokes. Preferred white mineral oils include those available from Witco Corporation, Arco Chemical Company, PSI and Penreco. Preferred paraffinic oils include API Group I and II oils available from Exxon-Mobil Chemical Company, HVI neutral oils available from Shell Chemical Company, and Group II oils available from Arco Chemical Company. Preferred MVI naphthenic oils include solvent extracted oils available from Equilon Enterprises and San Joaquin Refining, hydrotreated oils available from Equilon Enterprises and Ergon Refining, and naphthenic oils sold under the names HYDROCAL and CALSOL by Calumet, and described in U.S. Pat. No. 5,348,668 to Oldiges.

Dispersants

The ashless dispersants commonly used in the automotive industry contain an lipophilic hydrocarbon group and a polar functional hydrophilic group. The polar functional group can be of the class of carboxylate, ester, amine, amide, imine, imide, hydroxyl, ether, epoxide, phosphorus, ester carboxyl, anhydride, or nitrile. The lipophilic group can be oligomeric or polymeric in nature, usually from 70 to 200 carbon atoms to ensure oil solubility. Hydrocarbon polymers treated with various reagents to introduce polar functions include products prepared by treating polyolefins such as polyisobutene first with maleic anhydride, or phosphorus sulfide or chloride, or by thermal treatment, and then with reagents such as polyamine, amine, ethylene oxide, etc.

Of these ashless dispersants the ones typically used in the petroleum industry include N-substituted polyisobutenyl succinimides and succinates, allyl methacrylate-vinyl pyrrolidinone copolymers, alkyl methacrylate-dialkylaminoethyl methacrylate copolymers, alkylmethacrylate-polyethylene glycol methacrylate copolymers, and polystearamides. Preferred oil-based dispersants that are most important in the instant application include dispersants from the chemical classes of alkylsuccinimide, succinate esters, high molecular weight amines, Mannich base and phosphoric acid derivatives. Some specific examples are polyisobutenyl succinimide-polyethylenepolyamine, polyisobutenyl succinic ester, polyisobutenyl hydroxybenzyl-polyethylenepolyamine, bis-hydroxypropyl phosphorate. For instance, bis-succinimide is a dispersant based on polybutene and an amine which is suitable for oil based dispersions and is commercially available under the tradenames of INFINEUM C9231, INFINEUM C9232, and INFINEUM C9235 which is sold by Infineum, USA, L.P. The C9231 is borated while the C9232 and C9235 are not; however, all are bis-succinimides which differ due to their amine to polymer ratio.

The dispersant may be combined with other additives used in the lubricant industry to form a dispersant-detergent (DD additive package, e.g., LUBRIZOL[R] 9802A and/or the concentrated package (LUBRIZOL[R] 9802AC), which are mixed Dispersants having a high molecular weight succinimide and ester-type dispersant as the active ingredient, and which also contains from about to 9.9 percent by weight of zinc alkyldithiophosphate, from 1 to 4.9 percent by weight of a substituted phenol, from 1 to 4.9 percent of a calcium sulfonate, and from 0.1 to 0.9 percent by weight of a diphenylamine; wherein the whole DI package can be used as dispersing agent for the carbon nanomaterial dispersion.

Another preferred dispersant package is LUBRIZOL OS#154250 which contains from about 20 to 29.9 percent by weight of a polyolefin amide alkeneamine, from 0.5 to 1.5 percent by weight of an alkylphosphite, about 1.1 percent by weight of a phosphoric acid, and from 0.1 to 0.9 percent by weight of a diphenylamine, with primary active ingredient believed to be polyisobutenyl succinimides and succinates. Another preferred dispersant package is a high molecular weight succinimide DI package for diesel engines LUBRIZOL[R] 4999 which also contains from about 5 to 9.9 percent zinc alkyldithiophosphate by weight.

Other Types of Dispersants

Alternatively a surfactant or a mixture of surfactants with low HLB value (typically less than or equal to 8), preferably nonionic, or a mixture of nonionics and ionics, may be used in the instant invention.

The dispersant for the water based carbon nanomaterial dispersion, more specifically carbon nanotube dispersion, should be of high HLB value (typically less than or equal to 10), preferable nonylphenoxypoly(ethyleneoxy)ethanol-type surfactants are utilized.

The dispersant can be in a range of up from 0.001 to 30 percent, more preferably in a range of from between 0.5 percent to 20 percent, more preferably in a range of from between 1.0 to 8.0 percent, and most preferably in a range of from between 2 to 6 weight percent.

The carbon nanotube or graphite nanoparticles can be of any desired weight percentage in a range of from 0.0001 up to 50 percent by weight providing for an effective amount to obtain the desired thermal enhancement of the selected fluid media. For practical application an effective amount of carbon nanomaterials is usually in a range of from between 0.01 percent to 20 percent, and more preferably in a range of from 0.02 to 10 percent, and most preferably in a range of from between 0.05 percent to 5 percent. The remainder of the formula is the selected medium comprising oil, water, or combinations thereof together with any chemical additives deemed necessary to provide lubricity, corrosion protection, viscosity, or the like.

It is believed that in the instant invention the dispersant functions by adsorbing onto the surface of the nanoparticle material.

Other Chemical Compounds

This dispersion may also contain a large amount of one or more other chemical compounds, preferably polymers, not for the purpose of dispersing, but to achieve thickening or other desired fluid characteristics.

The viscosity improvers used in the lubricant industry can be used in the instant invention for the oil medium, which include olefin copolymers (OCP), polymethacrylates (PMA), hydrogenated styrene-diene (STD), and styrene-polyester (STPE) polymers. Olefin copolymers are rubber-like materials prepared from ethylene and propylene mixtures through vanadium-based Ziegler-Natta catalysis. Styrene-diene polymers are produced by anionic polymerization of styrene and butadiene or isoprene. Polymethacrylates are produced by free radical polymerization of alkyl methacrylates. Styrene-polyester polymers are prepared by first co-polymerizing styrene and maleic anhydride and then esterifying the intermediate using a mixture of alcohols.

Other compounds which can be used in the instant invention in either the aqueous medium or the oil medium include: acrylic polymers such as polyacrylic acid and sodium polyacrylate, high-molecular-weight polymers of ethylene oxide such as Polyox[R] WSR from Union Carbide, cellulose compounds such as carboxymethylcellulose, polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP), xanthan gums and guar gums, polysaccharides, alkanolamides, amine salts of polyamide such as DISPARLON AQ series from King Industries, hydrophobically modified ethylene oxide urethane (e.g., ACRYSOL series from Rohmax), silicates, and fillers such as mica, silicas, cellulose, wood flour, clays (including organoclays) and nanoclays, and resin polymers such as polyvinyl butyral resins, polyurethane resins, acrylic resins and epoxy resins.

Other chemical additives used in lubricants such as pour point depressant can also be used in the instant invention. Most pour point depressants are organic polymers, although some nonpolymeric substances have been shown to be effective. Commercial pour point depressants include alkylnaphthalenes, polymethacrylates, polyfumarates, styrene esters, oligomerized alkylphenols, phthalic acid esters, ethylenevinyl acetate copolymers, and other mixed hydrocarbon polymers. The treatment level of these additives is usually low. In nearly all cases, there is an optimum concentration above and below which pour point depressants become less effective.

Acrylic copolymers such as manufactured by Supeleo Inc. in Bellefonte, Pa. as ACRYLOID 3008 is a pour point depressant useful in the present invention.

Still other chemical additives used in lubricants, such as rust and oxidation inhibitors, demulsifiers, foam inhibitors, and seal-swelling agents can also be used in the instant invention. Physical Agitation.

The following trade names correspond to the chemical definition as follows:

Additive Description Yubase 4 Group III base oil Yubase 6 Group III base oil PAO 4 Group IV base oil LZ 21303 Passenger car detergent-inhibitor package LZ 8676 Antioxidant mixture LZ 8650 Organic friction modifier Afton 5777 dispersant viscosity modifier LZ 7749B PMA pour point depressant T503-209-2 Nano graphite concentrate Star 4 Group II base oil Star 8 Group II base oil LZ 20010 Passenger car detergent-inhibitor package LZ 6473 Calcium sulfonate detergent LZ 7075F OCP viscosity modifier A-11 Nano graphite concentrate

Milling Procedure:

Graphite particles were obtained by pulverizing big graphite chunks from the Carbide/Graphite Group, and size-selected through a mesh filter to be less than 75 μm. Thirty (30) grams of the above graphite particles and 270 grams of DURASYN 162 (a commercial 2 centistokes polyalphaolefin, abbreviated hereafter as 2 cSt PAO or preferably 4 csT PAO), were added into the EIGER Mini Mill (Model: M250-VSE-EXP). The milling speed was gradually increased to 4000 rpm. In about 4 hours the above mixture turned into thick paste. Sixty grams of this paste was discharged and labeled as Paste A. For the rest of the mixture in the mill, 48 grams of a dispersant was added and an additional dispersant inhibitor package (DI package) from Lubrizol, LUBRIZOL 9677MX was added into the mill and the paste became very thin, and successful recirculation was restored. The mill was stopped after another 4 hours of milling and the discharged paste was labeled as Paste B. Paste C was obtained by milling a mixture of 30 grams of graphite with diameter less than 75 μm, 60 grams of LUBRIZOL 9677MX, and 270 grams of DRASYN 162 at 4000 rpm for 8 hours. Note here the dispersing agent LUBRIZOL 9677MX was added into the mill at the very beginning. Three fluids A through C were formulated using the above three pastes as concentrates whereby their final composition were exactly the same: 2% graphite, 4% LUBRIZOL 9677 MX, 18% DURASYN 162, 76% DURASYN 166 (a commercial 6 centistokes polyalphaolefin, abbreviated hereafter as 6 cSt PAO) (all percentage by weight). Example 3 illustrates the 100° C. viscosity and thermal conductivity increase of the fluids.

It was also found that the graphite particle size before milling was very critical on the viscosity modification effect as well. For example, starting with graphite smaller than 10 (obtained as graphite powder from UCAR Carbon Company Inc.) and following the same procedure as Paste B, a thin Paste D was obtained. A fluid D was formulated with the same composition as fluid A and the result is listed in Example 3 as well. The particle size is measured by atomic force microscopy (AFM), and FIG. 2 illustrates an AFM picture of Fluid B. The graphite nanoparticles are plate-like structure, with average diameter is around 50 nm and thickness around 5 nm (as described earlier, nanodisks or nanoplates).

TABLE 1 Fluids and viscosity data from Example 1 Fluid A B C D From Concentrate Paste A Paste B Paste C Paste D Kinematic viscosity at 100° C., cSt 7.55 19.68 10.83 7.48 Kinematic viscosity at 40° C., cSt 28.44 29.32 28.77 27.85 Thermal Conductivity W/mK 0.18 0.19

It is important to note that without added dispersant the mixture of graphite and base oil turns into paste in hours and thus sufficient milling can not be accomplished. With good dispersant the milling can be extended to as long as desirable without paste formation. It is also important to adjust the temperature to maintain the right viscosity during the milling process so that the efficient milling is achieved.

To illustrate the importance of the milling process, and in minimizing the heating that occurs in dry milling we compared the thermal conductivity increase achieved using two dry processes and the wet process described in this invention. Sample “V 174-01” is obtained by milling “UCAR GS4-E” in PAO 2 with a polyisobutenylsuccinimide dispersant for 11 hours at 120° F. The UCAR GS4-E starting material has high bulk thermal conductivity, but due to its shape and size does not give the desired benefit in increasing fluid thermal conductivity. Thus the wet milling creates the particle shape that improves thermal conductivity and also avoids surface damage that can decrease thermal conductivity. The thermal conductivity (k) percent increase reported is compared to the base fluid (PAO) alone. The data are shown in Table 2 below:

TABLE 2 Thermal Conductivity Percent Increase Compared to Base Fluid Alone Graphite Source Way of Milling peak 1 peak 2 k increase @2%(%) Acheson SLA1275 dry(ball)  124 nm(33%)  497 nm(67%) 8.59 V 174-01 wet(horizontal)  117 nm(69%) 2413 nm(31% 29.27 UCAR GS4-E dry(jet) 3304 nm(100% 3.4

It is evident that both the particle size and the way of milling are important to thermal conductivity increase.

Results from the table show that the commercial Acheson sample generally has smaller particles than the sample V174-01 of graphitic material obtained from the instant process, however, the V174-01 sample has a much higher thermal conductivity boost. This surprising result is contrary to the expected relationship of increasing thermal conductivity with decreasing particle size, but it indicates the importance of the invention milling process in increasing thermal conductivity.

It is believed that in the dry ball milling process, local high temperature exists which could cause graphite surface fracture or surface defects, including oxidation which reduces thermal conductivity. For example, in U.S. Pat. No. 4,434,064 which issued in February of 1984 by Chao et al., a graphite dispersion was made (with larger graphite particles as compared to the instant invention) and surface oxidation caused by dry grinding in oxygen atmosphere is preferred because it aids in dispersion. In the instant invention, the wet milling process this effect is very much controlled and minimized. It is also important to begin with graphites developed in a process that preserves purity and crystalline structure.

One preferred starting material is jet-milled graphite. Jet mills have higher efficiency in producing ultra fine grade particles and they are claimed to be contamination free. The basic premise of the jet mill is to utilize the energy of compressed gas to perform the grinding. The gas accelerates the material, causing high-speed particle-on-particle collisions. As a result, the material grinds against itself, ensuring product quality. With the expansion of the compressed gas, a cooling effect takes place allowing heat-sensitive materials to be processed without degradation. However, without further milling in the solvents, the thermal conductivity boost is still very limited due to size and shape. For example, a 2% dispersion of jet-milled graphite has only 3.4% increase in thermal conductivity.

Scanning electron microscope pictures demonstrate this important shape change which occurs to the graphite particle shape with solvent milling. The raw material, UCAR GS4-E, is composed in the shape of chunks as best illustrated in FIG. 1. After the solvent milling, the chunks are processed and appear as a plate-like shape as shown in FIG. 2.

Carbon nanotubes, double wall, multi-wall or single wall having a controlled aspect ratio, are another preferred type of nanomaterial or particles. The nanotubes have a typical nanoscale diameter of 1-200 nanometers. More typically the diameter is around 10-30 nanometers. The length of the tube can be in submicron and micron scale, usually from 50 nanometers to 100 microns. More typical length is 500 nanometers to 50 microns. The aspect ratio of the tube (which is defined by the average length of the tubes divided by the average diameter) can be from hundreds to thousands, more typical 100 to 2000. The surface of the nanotube can be treated chemically to achieve certain level of hydrophilicity, or left as is from the production.

The nanoplates and nanotubes can be mixed to obtain desired viscosity/shear and thermal conductivity behavior. Other high thermal conductivity carbon materials are also acceptable as long as they meet the thermal conductivity and size criteria set forth heretofore.

To confer long-term stability, an effective amount of one or more chemical dispersants or surfactants is preferred, although the special milling procedure in base oil described heretofore will also confer long term stability. The thermal conductivity enhancement, compared to the fluid without graphite, is proportional to the amount of nanomaterials added, their thermal conductivity, and their size and method of dispersion. The particles of the instant invention will impart a thermal conductivity in fluid higher than the neat fluid, wherein the term ‘neat’ is defined as the fluid before the particles are added.

The concentration, size and shape of the graphite nanoparticles or nanotubes, along with the dispersant/surfactant type and concentration, is adjusted to provide the desired contribution to the overall fluid characteristics as for example the .viscosity and shear stability. Similar adjustments can be envisioned for producing composite resins or polymer melts to produce plastics.

It is observed that a bimodal distribution of particles is found in the finished sample. To reduce the large particle distribution recycling of the dispersion in the mill is necessary. A selected recycle ratio of to is desirable. Furthermore the large particles in the final material can be removed by a filtration or centrifugation step.

High-Temperature Shear Stable Graphite Dispersion.

According to the Albert Einstein equation regarding the viscosity of dispersions, a good dispersion just has slight viscosity increase while a bad dispersion has a big viscosity increase. The viscosity ranges of many fluids are very critical. So it is very important to not cause viscosity change while integrating nano particles into many fluids. The particles can be dispersed in oil by choosing the right dispersants, typically fairly low molecular weight materials (<2500 m. w.) such as polysuccinimides, and somewhat higher molecular weight viscosity index improvers, with dispersant functionality. To determine good or bad dispersions rheometer are used instead of viscosity tubes. The rheometer measures viscosities under varying shear stress. For a Newtonian fluid the viscosity is a constant regardless of shear rate. Many fluids are non-Newtonian fluids, but at low shear rates the viscosity is also a constant. For bad dispersions the viscosity shows shear-thinning with a increase in shear rate and builds up at high temperature as time pass by. As set forth in FIG. 3, repeatable almost flat lines in the rheometer plots of viscosity are comparable against the shear rate for well dispersed graphite oils shown in FIG. 4.

The viscosity ranges of many fluids are very critical. So it can be very important to not cause viscosity change while integrating nano particles into many fluids. By choosing the right viscosity improver, “VI”, (polymers known to the lubricants industry as viscosity index improvers), dispersions can be improved. The VI improvers we used are dispersant VI improvers.

To determine good or bad dispersions a rheometer is used instead of viscosity tubes. Rheometer measures viscosities under shear stress which is variable. For a Newtonian fluid the viscosity is a constant regardless of shear rate. Motor oils are non-Newtonian fluids, but at low shear rates the viscosity is also a constant. For bad dispersions the viscosity shows shear-thinning with a increase in shear rate and builds up at high temperature as time passes by as exhibited in FIG. 3. Repeatable, almost flat lines in the rheometer plots of viscosity against the shear rate for well dispersed graphite oils as shown in FIG. 4.

Table 3 shows how increasing the percentage of carbon nanotubes in oil results in a much greater increase in viscosity as compared with increasing the percentage of carbon nanographite particles of the instant invention due to the particle shape which is changed during the milling process.

TABLE 3 Viscosity Increase with Weight Percentage of nanotube and Nanographite % Carbon Viscosity 100° % Milled Viscosity 100° Nanotube C./cST Nanographite C./cST 0 3.92 0 3.92 0.005 4 0.01 5.36 0.05 8.59 0.1 16.75 0.1 4.04 0.2 65 0.2 4.19

The following examples provide formulations of compositions in accordance with the present invention and provide examples of the range of ingredient percentages by weight providing an effective amount of the particular ingredients deemed necessary to obtain the desired results in single application.

Example 1 Synthetic Lubricant Composition Containing Nanographite Particles

Synthetic SAE 5W-30 Yubase 4 39.43 Yubase 6 25.00 PAO 4 10.00 LZ 21303C 11.00 LZ 8676 1.00 LZ 8650 3.40 Afton 5777 3.60 LZ 7749B 0.40 T503-209-2 (Cut 2) 9.17 V100 Before V100 After 9.88 Delta V CCS@−30 C. 5361 NOACK, % Loss 12.08 MRV @ −35 C. YS <35 Viscosity 17022 HTHS

Example 2 Conventional Lubricant Composition Containing Nanographite Particles

Conventional SAE 5W-30 Yubase 4 50.00 Star 4 8.59 Star 8 20.50 LZ 20010 10.85 LZ 8650 0.30 LZ 8676 0.30 LZ 6473 0.50 Afton 5777 1.00 LZ 7075F 4.60 LZ 7749B 0.40 A-11 2.96 100.00 KV (100 C.)bfr KV (100 C.) aftr CCS (−30 C.) NOACK After Bosch

Example 3 Semi Synthetic Lubricant Composition Containing Nanographite Particles

Ingredients Percentage range Graphite 0.1-2   Group III and IV base oils 80.0-85.0 LZ DI package  7.0-15.0 LZ additives 0.1-2.0 Viscosity improver 1.0-7.5 Viscosity at 100° C.  7.5-15.0

Table 4 shows that adding nanomaterials into a lubricant can significantly increase the thermal conductivity of the formulation, which implies better thermal management for the system.

For example in Table 4, a semi-synthetic blended motor oil such as DURABLEND which is sold by Valvoline Inc., a division of ASHLAND INC. is compared as DB (a conventional 5W-30 motor oil), NF-1 (a 5W-30 motor oil containing graphite nanoplates), and NF-2 (a 5W-30 motor oil containing graphite nanoplates and carbon nanotubes) showing the effect on viscosity index and thermal conductivity K(w/m·K).

Code DB NF-1 NF-2 Product Conventional DuraBlend DuraBlend 5W-30 with DuraBlend 5W-30 with graphite description 5W-30 graphite nanoplate nanoplate and carbon nanotubes Percent by wt. 0 1.0, graphite nanoplate* 1.0, graphite nanoplate* Nanomaterial 0.1, carbon nanotubes** Vis 100° C. 10.66 10.9 10.9 Vis at 4° C. 61.14 57.1 54.34 Viscosity Index 166 186 197 k (w/m · K) 0.1423 0.1591 0.1768 *Graphite is obtained as carbon fibers from Union Carbide and further processed through in-house method into graphite nanoplate. **Multiwalled carbon nanotubes are obtained from University of Kentucky.

The foregoing detailed description is given primarily for clearness of understanding and no unnecessary limitations are to be understood therefrom, for modification will become obvious to those skilled in the art upon reading this disclosure and may be made upon departing from the spirit of the invention and scope of the appended claims. Accordingly, this invention is not intended to be limited by the specific exemplifications presented herein above. Rather, what is intended to be covered is within the spirit and scope of the appended claims. 

1. A product made from a process for producing a nanographite dispersion in a fluid having the thermal conductivity of the dispersion enhanced from the base fluid by more than 10% for a 1% graphite dispersion, comprising the steps of: selecting a high purity graphite with high crystallinity, reduced surface damage, and reduced oxidation as the starting material; wet media milling said starting material in the presence of a dispersant and a solvent; controlling a mill temperature and atmosphere to control and reduce surface damage to said starting material; milling until a desired nanographite with flake shape and controlled aspect ratio is achieved; milling until a particle size average of 300 nm diameter and 50 nm thick is or smaller is obtained; recycling a portion of the milled material to increase the ration of small particle distribution to large particle in an intermediate product with small and large particle bi-modal distribution; removing the large particle distribution from the finished product by centrifugation or filtration, and adding said intermediate product to a lubricant.
 2. The product by process of claim 1, wherein said mill is selected from a dry ball mill, a wet ball mill, and a jet mill.
 3. The product by process of claim 1, wherein the dispersant is an ashless dispersant.
 4. The product by process of claim 1, wherein the dispersant is selected from the group consisting of a lipophilic hydrocarbon group and a polar functional hydrophilic group wherein the polar functional group comprises a carboxylate, ester, amine, amide, imine, imide, hydroxyl, ether, epoxide, phosphorus, ester carboxyl, anhydride, and nitrile, and the lipophilic group comprises an oligomeric or polymeric compound from 70 to 200 carbon atoms to ensure oil solubility and hydrocarbon polymers treated with various reagents to introduce polar functions including products prepared by treating polyolefins such as polyisobutene first with maleic anhydride, or phosphorus sulfide or chloride, or by thermal treatment, and then with reagents such as polyamine, amine, and ethylene oxide.
 5. The process of claim 1, wherein the dispersant is selected from the group consisting of a N-substituted polyisobutenyl succinimides and succinates, alkyl methacrylate-vinyl pyrrolidinone copolymers, alkyl methacrylate-dialkylaminoethyl methacrylate copolymers, alkyl methacrylate-polyethylene glycol methacrylate copolymers, and polystearamides. Preferred oil-based dispersants that are include dispersants from the chemical classes of alkylsuccinimide, succinate esters, high molecular weight amines, MANNICH base and phosphoric acid derivatives. Some specific examples are polyisobutenyl succinimide-polyethylenepolyamine, polyisobutenyl succinic ester, polyisobutenyl hydroxybenzyl-polyethylenepolyamine, bis-hydroxypropyl phosphorate, LUBRIZOL 890 (an ashless PIB succinimide), LUBRIZOL 6420 (a high molecular weight PIB succinimide), ETHYL HITEC 646 (a non-boronated PIB succinimide), a PIB Succinimide, and a dispersant VI improver ETHYL
 5777. 6. The product by process of claim 1 wherein said starting material comprises a pasty liquid of particles with mean size less than 500 nanometers in diameter and having a range of from 100 to 500 nm in diameter and from 20 to 80 nm in thickness. 